It is known within the oil- and gas-drilling industry to take measures to be able to maintain a circulation of drilling fluid in the borehole while a drill string is being lengthened. NO 326427 discloses a system in which a top-drive drilling machine with a hollow drive shaft cooperates with a gate chamber provided with seals surrounding the pipe, in which the drilling fluid is passed alternately through the drive shaft via a first fluid inlet, when a continuous drill string is connected to the drilling machine, and through the gate chamber via a second fluid inlet, when the upper end of the pipe string is arranged in the gate chamber and is to be joined to a new pipe section.
A drawback of the prior art of NO 326427 is that the pipe-string rotation ceases when the pipe string is to be lengthened. It is known in the industry that it is an advantage to maintain the pipe-string rotation both to reduce the risk of the pipe string sticking and to improve the productivity, for example increase the drilling capacity by there being no stop in the actual drilling operation while the drill string is being lengthened.
From the NO patent 333021 and the corresponding WO publication 2011/093716, an arrangement is known, in which, between a first top-drive drilling machine and a borehole, are arranged a second drilling machine provided with a rotary table arranged to take the weight of a pipe string, a rotary-drive unit arranged to continuously rotate the pipe string, and a fluid chamber which is arranged to connect a pipe-string end portion in a fluid-communicating manner to a drilling-fluid system, the fluid chamber being provided with pipe-string ports including means arranged to close the pipe-string ports in a fluid-sealing manner, and the second drilling machine further being provided with a pair of power tongs which is arranged to connect/disconnect an element to/from the pipe string, the power tongs being arranged in the fluid chamber. The drawback of this arrangement is that the rotary-drive unit is directly connected to the fluid chamber (gate chamber) and the power tongs are enclosed in the fluid chamber. Here, the adjustment of the power tongs to the relevant pipe dimension that is being handled must take place by intervention in the fluid chamber.
From WO0169034 A2, a system is known, for the continuous circulation of fluid to and through a pipe string while an upper pipe is connected to or removed from the upper end of the pipe string. The system includes upper and lower chambers, each provided with a sealing device, which is arranged to rest sealingly against a portion of the pipe string, and with an intermediate gate apparatus.
WO 2008/147210 A2 discloses a device for a top-drive drilling machine, in which a continuous circulation of drilling fluid through a drill string may be maintained by the drilling fluid being supplied alternately through a chamber, fitting tightly against an upper portion of the drill string, and the drilling-machine drive shaft when the lower end portion thereof or of a connected pipe section opens into the chamber.
US 2003/0221519 A1 discloses an apparatus allowing the connection or disconnection of pipes relative to a pipe string during a drilling operation. The apparatus also allows the rotation and axial displacement of the pipe during the connection and disconnection operations and the circulation of drilling fluid. A top-drive drilling machine cooperates with a rotary table, and the drilling-fluid circulation is provided alternately through a circulation unit and through the top-drive drilling machine and the connected, upper pipe.